This is a quick introduction to the topic of choosing the best Non Invasive Ventilation (NIV) mode to treat a patient in Acute Respiratory Failure (ARF).
Selecting the most apropriate mode of non-invasive ventilation depends on the catagory of respiratory failure.
Acute respiratory failure can be divided into two catagories based on the root cause:
Type 1. Hypoxaemic.
Failure to oxygenate.
Problems with movement of oxygen from the lungs to the blood. Normally associated with issues in the difusion of oxygen from the alveoli to the pulmonary circulation.
Type 2. Hypercapnic.
Failure to ventilate.
Problems with moving gas in and out of the lungs.
Characterised by increased arterial carbon dioxide (CO2) levels.
Usually due to inadequate spontaneous ventilation from:
- decreased respiratory drive
- increased work of breathing (WOB).
- problems with airway conduction.
Some examples include:
Once the underlying problem has been identified the best mode of non invasive ventilation can be selected:
Provides one single selected pressure into the patients mask throughout their respiratory cycle.
On inspiration there will be 10 cmH2O pressure supplied to the patients mask.
As the patient breathes in, this addditional pressure in the mask that is ‘sealed’ to the patients face will assist with inspiration (ie decrease his work of breathing or WOB)
The increased pressure in the mask will flow all the way down the airway to the alveoli. This will fill alveoli that may otherwise be partially collapsed (recruitment), increasing the surface area that is available for gas exchange.
On expiration there will still be 10 cmH2O pressure supplied to the patients mask.
The patient is now breathing out against the resistance of this pressure. The positive pressure will now ‘hold’ those alveoli open at the end of expiration increasing the time available for gas exchange (known as: increased functional residual capacity).
This positive pressure is also known as Positive End Expiratory Pressure or PEEP.
So, CPAP = PEEP.
Bi-Level ventilation (or BiPAP)
Two separtate pressures are selected.
One for inspirataion (IPAP).
One for expiration (EPAP).
This provides the benefits of CPAP with the additional benefit of an increased support during inspiration. This extra support above the setting for CPAP is called: Pressure Support.
For example, if the doctor was to select IPAP=15 and EPAP=10:
On inspiration there will be 15 cmH2O pressure supplied to the patient mask.
This higher pressure will provide an incresed support as the patient breathes in. Further decreasing WOB (and therefore resp muscle fatigue and myocardial oxygen demand).
This incresed pressure also encourages the patient to take bigger breaths (or Tidal Volumes) which helps remove any excess CO2 from the bloodstream.
On exhalation there will be a lesser pressure of 10 cmH2O supplied to the patients mask.
So it will be a little easier to breathe out for the patient. But there will still be a resistance and a pressure at the end of exhalation with the same effects as those of CPAP.
Pressure Support: As the inspiratory (IPAP) pressure is 5 cmH2O above the expiratory (EPAP) pressure the pressure support is said to be 5 cmH2O.
So, we have : IPAP – EPAP = Pressure Support (PS)
Increasing pressure support by widening the distance between IPAP and EPAP will decrease the patients WOB.
Increasing EPAP ( which is the same thing as PEEP) will assist to increases the patients arterial oxygen concentration.
So in summary, selection of the best mode of NIV is dependant on the type of respiratory failure experienced by the patient:
NOTE: This article is not intended to be a comprehensive teaching of the principles of Non-Invasive ventilation, CPAP or BiPAP. It is a quick explanation aimed at those having diffiuclty with first principles of this topic. For more detailed information see: http://lifeinthefastlane.com/ccc/non-invasive-ventilation-niv/